UDMF: Use string values for the sector "triggerer" field
The triggerer sector field, which is an enum, is currently implemented as an integer within the TEXTMAP. I originally did it this way to make it more comfortable to work with in UDB: Since it's listed as a custom field, the idea was supply a list of possible enum values via the config. This ended up not working. Now that I'm working on the interface for UZB, I realize that all the UDMF enum fields in ZDoom (such as damagetype) are implemented as strings. Besides being easier to support from the map editor side, this is also better for forward compatibility from the game side, so let's follow suit.